United Nations officials have set off to the Syrian town of Tremseh to investigate reports of a massacre of civilians by Syrian troops and pro-government militia.

A three-car convoy was seen leaving Damascus and heading for Tremseh and UN officials confirmed the destination to the Spanish Efe news agency.

The mission follows comments by the head of the UN monitoring mission in Syria that helicopters and tanks were used to shell Tremseh at the same time as villagers reported the killing of around 140 people.

General Robert Mood said on Friday his officials were ready to enter the town of Tremseh if a truce there took hold. He said monitors stationed in the area had personally verified that heavy weapons were directed against Tremseh on Thursday.

Late on Friday the UN said observers had been unable to contact the military commander responsible for the area near Tremseh and its members had been refused access. It claimed the Syrian air force "continued to attack populated urban areas on a large scale" and said its observers had logged more than 100 explosions in an "ongoing military operation".

Residents of the small town of around 6,000 people said they were attacked from outlying areas late on Thursday after many hours of shelling. They blamed the attack on loyalist forces and a pro-regime militia known as the Shabiha which has been accused of being at the vanguard of other mass killings in recent months.

Syrian officials instead blamed "terrorists", who they say have been responsible for much of the violence in Syria since the uprising began almost 17 months ago.

The UN's special envoy to Syria, Kofi Annan, said he was "shocked and appalled" by "intense fighting, significant casualties, and the confirmed use of heavy weaponry such as artillery, tanks and helicopters" in Tremseh, which is in an ethnically mixed area 22 miles from Hama.

Activists in the area say around 600 casualties are being treated in field clinics and hospitals in surrounding areas. They claim that many bodies remain in crop fields and a final death toll will not be known for days.

Mousab al-Hamadee, an opposition activist living 12 miles away, said people had travelled to the town early on Thursday to warn of an impending regime assault. Hamadee said he had spoken to his sister, who described the attack as relentless. "A big number of the young men were killed in the field when they were trying to escape the army attack," he said. "Helicopters targeted them by heavy machine guns while they were driving their motorcycles – while they were fleeing the village.

"Today the people of Tremseh opened a house that was burned by troops. They found two people who were burned alive. My sister told me that the only two doctors in the village were targeted by mortar shells. Both doctors were killed in their houses."

The alleged massacre took place before a UN security council meeting that is expected to weigh a new response to the crisis in Syria, which has morphed over the past year from a series of anti-regime demonstrations in many towns and cities into a full-scale insurrection.

Some European states and the US are pushing for the UN to impose sanctions under a Chapter Seven resolution. However, Russia and China, which have staunchly supported Damascus throughout the uprising, have indicated that they will use their vetoes to block such a move.

Damascus said earlier this week it was committed to Annan's six-point peace plan and nominated an interlocutor that it said would represent the regime in discussions with the opposition to bring about a ceasefire.